Method and apparatus for efficient transfer of data over a network

ABSTRACT

A method and apparatus for improving the efficiency of a network. A source node determines the propagation delay for data to reach a destination node. This enables the source node to transmit data more efficiently by ensuring a greater portion of a specific time slot is used for receiving data by the destination node. The destination node then determines if it is connected to any other nodes, and determines the propagation delay between the other connected nodes. The process continues until a node detects it is not connected to any other nodes for which a propagation delay has not been computed. Thus, each node on the network knows the propagation delay between each node, and the nodes utilize this information to more efficiently transfer data through the network.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This application is a continuation of U.S. patent application entitled“METHOD AND APPARATUS FOR EFFICIENT TRANSFER OF DATA OVER A NETWORK”,having Ser. No. 11/311,452, and filed Dec. 19, 2005, which isincorporated by reference herein.

Embodiments of the present invention generally relate to a network and,more specifically, to the efficient transfer of data through a network,as well as techniques for monitoring and management of a network.

2. Description of the Related Art

A conventional network is comprised of nodes and a router, typicallyarranged in a hub and spoke formation. A node communicates to anothernode by transmitting data to the router, and the router then couples thedata to the destination node. The nodes are unaware of the presence ofall the other nodes present on the network, but the router is aware ofthe presence of all the nodes on the network. The router stores thephysical address of all the nodes on the network and facilitates thetransmission of data from one node to another.

An ad-hoc network differs from a conventional network because there isno central switching point, i.e., a router, to distribute the datawithin the network. The nodes are aware of the presence of other nodeson the network, and data is transmitted across the network by passingthe data through interconnected nodes.

One example of an ad-hoc network is a mesh network. A node on a meshnetwork receives and transmits data only during specific time slots. Forexample, a source node transmitting data to a destination node transmitsdata during a specific time slot designated for listening for data atthe destination node. The time slot is designed with a length comprisinga data portion and a portion allocated for propagation delay. Thus, thepropagation delay and the distance between the source and destinationnode causes a portion of the time slot to be unutilized and thetransmission of data is not as efficient as possible.

In a network where the distance between source and destination isunknown, the time slot must accommodate the worst case propagationdelay. To avoid such transmission inefficiency, the propagation delaymay be measured by using time synchronized nodes and sending time oftransmission information between nodes. Thus, any node can compute thepropagation delay by subtracting the current time from the time oftransmission. The time slot length can then be customized to conform tothe computed delay such that an entire time slot can be used for datatransmission. However, to facilitate such a computation, the network issynchronized to a time base such as the Global Positioning System (GPS)or some other universal time base. Such synchronization adds cost todeveloping and deploying a network.

Therefore, there is a need in the art for an improved method andapparatus for efficiently transferring data through a time divisionmultiple access network.

SUMMARY OF THE INVENTION

The present invention is a method and apparatus for improving theefficiency of a network and/or providing network monitoring andmanagement capability. In one embodiment of the invention, a source nodedetermines the propagation delay for data to reach a destination node.This enables the source node to transmit data more efficiently byensuring a greater portion of a specific time slot is used for receivingdata by the destination node. The destination node then determines if itis connected to any other nodes, and determines the propagation delaybetween the other connected nodes. The process continues until a nodedetects it is not connected to any other nodes for which a propagationdelay has not been computed. Thus, each node on the network knows thepropagation delay between each node, and the nodes utilize thisinformation to more efficiently transfer data through the network and/orto monitor and manage the network.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a graphical view of a network;

FIG. 2 is a block diagram of a node of a network;

FIG. 3 is a flowchart detailing a method of implementing the presentinvention;

FIG. 4 is a graphical depiction of ranging using the present invention;

FIG. 5 is a flowchart detailing a method of calibrating a clock that canbe implemented within the present invention; and

FIG. 6 is a graphical depiction of triangulation between three nodes todetermine the location of a fourth node.

DETAILED DESCRIPTION

FIG. 1 is a graphical depiction of one embodiment of a network 100comprising connected nodes 102. The nodes 102 are interconnected to eachother such that multiple paths (represented by lines between nodes)exist between each node 102. An exemplary network that may utilize andbenefit from the present invention is a mesh network disclosed incommonly assigned U.S. patent application Ser. No. 10/122,883, filedApr. 15, 2002 and commonly assigned U.S. patent application Ser. No.10/122,886, filed Apr. 15, 2002, both of which are incorporated hereinby reference. Those skilled in the art will realize from the followingdisclosure that other network topologies, both wired and wireless, mayfind benefit by using the present invention.

The present invention is a method and apparatus for improving theefficiency of a network 100. In one embodiment of the invention, asource node 102 _(x) (where x is an integer), for example, node 102 ₁determines the propagation delay for data to reach a destination node102 _(y) (where y is an integer), for example, node 102 ₂. This enablesthe source node 102 ₁ to transmit data more efficiently by ensuring agreater portion of a specific time slot is used for transmitting data tothe destination node 102 ₂. The destination node 102 ₂ then determinesif it is connected to any other nodes 102 ₄, 102 ₅, 102 ₆, anddetermines the propagation delay between the other connected nodes. Theprocess continues until a node 102 _(x,y) detects it is not connected toany other nodes 102 _(x,y) for which a propagation delay has not beencomputed. Thus, each node 102 _(x,y) on the network knows thepropagation delay between itself and each neighboring node, and thenodes utilize this information to more efficiently transfer data throughthe network.

FIG. 2 is a block diagram of a node 102. The node 102 may be a one ormore individual computers, wireless devices, personal digitalassistants, desktop computers, laptop computers, servers or any otherdigital device that may benefit from connection to a computer orcommunications network. In one specific embodiment, one of the nodes,for example, node 102 ₁, is a gateway that connects the network 100 to abroadband backbone. As such, the gateway becomes the portal for networkdata traffic to locations outside of the immediate network 100.

The node 102 comprises a CPU 202, support circuits 206, memory 204 and anetwork interface 208. The CPU 202 may comprise one or more readilyavailable microprocessors or microcontrollers. The support circuits 206are well known circuits that are used to support the operation of theCPU and may comprise one or more of cache, power supplies, input/outputcircuits, network interface cards, clock circuits, and the like. Memory204 may comprise random access memory, read only memory, removable diskmemory, flash memory, optical memory or various combinations of thesetypes of memory. The memory 204 is sometimes referred to as main memoryand may, in part, be used as cache memory or buffer memory. The memory204 stores various forms of software and files, such as, an operatingsystem (OS) 210 and communication software 212. The communicationssoftware 212 is executed by the CPU 202 to facilitate operation of theinvention as described below. The operating system 210 may be own of anumber of well-known operating systems such as LINUX, WINDOWS, WINDOWSCE, SMARTPHONE, PALM, and the like.

The network interface 208 connects the node 102 to the network 100. Thenetwork interface 208 may facilitate a wired or wireless connection toother nodes. If the node 102 is a gateway, then the network interface208 couples to a wired broadband backbone. In addition, the interface208 couples through narrower bandwidth channels to other nodes of thenetwork via wired and/or wireless connections.

In one embodiment of the invention, the communication software 212 isused to increase the efficiency of a network that employs a timedivision multiple access architecture (TDMA). The invention findsparticular use in asynchronous networks where the nodes of the networkdo not rely on a common synchronous time base, e.g., GPS. Within anasynchronous, TDMA network, a source node, for example, node 102 ₁,transmits data during an allocated timeslot to a destination node, forexample, node 102 ₂. The destination node 102 ₂ receives or listens fordata during an allocated time slot. Thus, the efficiency of the network100 can be increased by the source node 102 ₁ increasing the probabilitytransmitted data is received at the beginning of a timeslot when thedestination node 102 ₂ is set to receive data, i.e., the transmit andreceive slots should be synchronized to maximize data throughput.

The probability that a source node 102 ₁ will transmit data in a mannerthat a destination node 102 ₂ will receive the data at the beginning ofa timeslot is affected by the propagation delay. As is well-known,propagation delay is the time lag between the departure of transmitteddata from a source node 102 ₁ and the arrival of the transmitted data ata destination node 102 ₂.

In one embodiment of the invention, the communication software 212executed by each of the nodes measures the propagation delay between thesource node 102 ₁ and a destination node 102 ₂ by performing a three-wayhandshake. Once determined, the source node 102 ₁ then transmits thepropagation delay to the destination node 102 ₂. The destination node102 ₂ uses the delay to delay the timeslot in which the destination node102 ₂ will listen for a transmission from the source node 102 ₁. Statedin another way, the delay is the amount of time prior to a destinationnode listening for data on an allocated timeslot that a source nodeshould transmit data to ensure the data is received at the beginning ofa timeslot allocated for receiving at the destination node.

Once the source node 102 ₁ and the destination node 102 ₂ are aware ofthe propagation delay between each other, each node can time thetransmission of data in such a manner that the receiving node receivesthe data at the beginning of a timeslot allocated for receiving data. Inthis manner, synchronization between the two nodes is achieved without acommon time base.

The destination node 102 ₂ then acts as a source node 102 ₂ vis-à-visthe neighboring nodes 102 ₄, 102 ₅, 102 ₆ on the network 100.Communication software 212 of the nodes determines the propagation delaybetween the destination node 102 ₂ and the connected nodes 102 ₄, 102 ₅,102 ₆. The process is used recursively throughout the network 100 in amanner that each node 102 is aware of the propagation delay betweenitself and any other connected node 102.

Consequently, timing information is propagated outwards through thenetwork 100 so that all nodes 102 are synchronized with their neighbornodes without using a common time base.

FIG. 3 depicts a flow chart of a method 300 of operation of thecommunication software 212 and FIG. 4 depicts a graphical view of thetransmission and reception of a data frames between two nodes 102 _(x)and 102 _(y). To best understand the operation of the invention, thereader should simultaneously view FIGS. 3 and 4.

The method 300 of FIG. 3 is divided into the method steps 301 performedby the source node 102 _(x) and the method steps 303 performed by thedestination node 102 _(y). The method 300 begins at step 302 andproceeds to step 304 wherein a source node 102 _(x) transmits asynchronization message 408 within a data frame 402. The message 408contains a time of transmission t₁. The transmission is addressed todestination node 102 _(y). The unknown propagation delay fortransmission of the data frame 402 from node 102, to node 102 _(y) isrepresented by reference number 406.

At step 306, the destination node receives the frame 402. At step 308,the destination node 102 _(y) stamps the message with its own timestamp409 provided by the network interface 208. The timestamp 409 indicatesthe local time, t₂, at which point the destination node 102 _(y)received the complete frame of data.

In one embodiment, the hardware used in the nodes is IEEE 802.11xcompliant, where x is a, b, c, d, e, f, or g. In such complianthardware, the time synchronization function (TSF) may be used to createthe timestamps based on the local time at the node. The conventionalusage of TSF is to synchronize the clock of the destination node 102_(y) with the clock of the source node 102 _(x). The invention does notrequire clock synchronization between the source node 102 _(x) and thedestination node 102 _(y) to function. However, the invention will stillfunction if the TSF is used to synchronize the clock of the destinationnode 102 _(y) with the clock of the source node 102 _(x). The operationof TSF is well-known by those skilled in the art.

Messages comprising timestamp information can also be used to calibratethe clock of the destination node 102 _(y) with the clock of the sourcenode 102 _(x). Calibration is performed by a source node 102 _(x)sending a plurality of messages comprising timestamp information to adestination node 102 _(y); the destination node 102 _(y) stamps themessage with a local timestamp indicating time of receipt, and thencalculates the difference between the time the source node 102 _(x)transmitted the message and the time of receipt. The method is repeatedfor each message within the plurality of messages sent by the sourcenode 102 _(x). The destination node 102 _(y) averages the differencesand uses the average to calibrate its clock.

At step 310, the destination node 102 _(y) stamps the message withtimestamp 410 containing the transmission time, t₃. Then, at step 312,the destination node transmits a data frame 404 to the source node 102_(x). The data frame 404 comprises the timestamps 408, 409, and 410 thatprovide the time of transmission of the data frame (t₁), the timereception was complete (t₂) and the time of return transmission of thedata frame (t₃).

At step 314, the source node 102 _(x) receives the data frame 404 and,at step 316, the node 102 _(x) determines the local time of completereception of the data frame, time t₄. At step 318, the source node 102_(x) computes the range between the source node 102 _(x) and thedestination node 102 _(y) based upon the timestamp information withinthe message and the time t₄. The source node knows the time oftransmission (t₁), the time of reception (t₂ minus the length of thedata frame), the time of transmission from the destination node (t₃) andthe time of reception at the source node (t₄ minus the length of thedata frame). These times are processed to derive the propagation delayas follows:

Propagation delay=(t₄−t₁−(t₃−t₂))/2

Calculation of the propagation delay can be continually repeated, and anaverage of the calculated propagation delays taken over time to increasethe resolution of the clock. For example, if the clock resolution usedis one microsecond, then the accuracy of one calculation isapproximately one microsecond, or one-fifth of a mile. By subtlyrandomizing the time that the messages are sent, messages aredecorrelated with the microsecond clock, and the ranging results willmost likely fall between two values, with the respective distributionbetween the values representing the true value. For example, if thecalculation yields results of 8,8,8,7,8,8,7,8,8,8 microseconds, then thetrue value is approximately 7.2 microseconds. This allows a moreaccurate result to be obtained beyond the clock resolution.

The implemented algorithm also takes into consideration the amount oftime it takes to send the frame (x bytes at y mbps), since in oneembodiment of the invention, the transmitted and received timestampscorrespond to the start-of-frame and end-of-frame.

The following steps are optional, i.e., these steps are not necessaryfor the invention to function, but may provide additional benefits. Atstep 320, the source node 102 _(x) transmits the range and/or thepropagation delay to the destination node 102 _(y). At step 322, thedestination node 102 _(y) receives the range (or delay). At step 324,the destination node 324 sets the frame timing for reception of datafrom the source node 102 _(x) such that data frames are efficientlyutilized.

At step 326, the destination node 102 tests to see if it is connected toany other nodes on the network 100 exclusive of the source node 102_(x). If the query is affirmatively answered, then the method proceedsto step 304 where the destination node becomes a source node vis-à-visthe other neighboring nodes. If the query is negatively answered, themethod ends at step 320.

FIG. 5 depicts a flowchart detailing a method of synchronizing andcalibrating a clock in a destination node with a clock in a source node.The method utilizes the standard 802.11 time synchronization function.

The method 500, starts at step 502 and proceeds to step 504. At step504, a source node sends a time synchronization function message to adestination node. At step 506, the destination node synchronizes itsclock to the source node in accordance with the standard 802.11 timesynchronization function. At step 508, the range between the source nodeand the destination node is calculated. At step 510, the clocks of thesource node and the destination node are misaligned by an amount equalto the propagation delay between the two nodes, allowing framestransmitted from the source node to the destination node to be receivedat a proper time.

The method 500, also provides a method for calibrating the clocksbetween a source node and a destination node. At step 512, the sourcenode transmits a plurality of one-way messages to the destination nodeto calibrate the clock at the destination node. At step 514, the clockspeed difference between the source node and the destination node iscomputed at the destination node. Ideally, because the clock of thesource node has been synchronized with the clock of the destination nodeand offset by the propagation time between nodes, the received timestampat the destination node should match the transmitted timestamp from thesource node. The clock drift between the source node and the destinationnode can be minimized by computing the clock speed difference betweenthe nodes and using the difference to calibrate the clock of thedestination node. The step of calibrating occurs in-between thesynchronization of the destination node with the source node. The methodends at step 516.

FIG. 6 depicts an arrangement of nodes that can be used to determine alocation of a node 608 by using directional antennas 616 ₁ to 616 ₃(collectively 616) located respectively on nodes 602, 604, and 608. Theantenna 616 comprises a plurality of radiators that form individual mainbeams having a substantially conical radiation pattern that radiallyextends from each antenna. The directional antennas 616 are designed toreceive and transmit signals within a cone-shaped area 610, 612, and 614such that the node 608 broadcasts a signal to each node 602, 604 and 606and a signal is received by the directional antennas 616 located on eachnode. Consequently, if the antenna 616 has six elements, each elementforms a radiation pattern that is approximately 30 degrees wide.

An approximate location of the node 608 can be determined using only afirst node 602 and a first directional antenna 616 ₁. The distancebetween the node 608 and the first node 602 is calculated using aranging technique as discussed above. The node 608 communicates withnode 602 via one antenna element (e.g., via cone 614). Thus, it is knownthat the node 608 must be located within the cone at the computed range.A more accurate location of the node 608 can be determined by narrowingthe focus of the directional antenna 616 ₁ depicted by the cone 614,e.g., adding more elements that have a narrower beamwidth.

A second node 604 can be used to establish an even more accuratelocation of the node 608. The second node 604 also has a directionalantenna 616 ₂. The range between the second node 604 and node 608 iscalculated. The node 608 is located within an area depicted by cone 610that radiates outward from the directional antenna 616 ₂ and intersectswith the area depicted by cone 614. An approximate location can bedetermined by finding locations within the intersection of cones 610 and614 that are within the calculated range from the first node 602 and thesecond node 604.

A unique location of the node 608 can be determined by using a thirdnode 606. The third node 606 has a directional antenna 616 ₃. The node608 is located within an area depicted by cone 612 that radiates outwardfrom the directional antenna 616 ₃ and intersects with the areasdepicted by cones 610 and 614. A unique location of node 608 can bedetermined using a triangulation technique by finding the locationwithin the intersection of cones 610, 612, and 614 that is within thecalculated range of nodes 602, 604, and 606.

In an alternative embodiment of the invention, the antennas 616 ₁ to 616₃ may be omni-directional antennas such that the ranging informationgenerated using the above-described method of node range determinationcan be used in a conventional triangulation computation to determinenode locations.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

The invention claimed is:
 1. A computer-implemented method forcalibrating a clock of a node comprising: synchronizing a destinationnode with a source node using a standard 802.11 time synchronizationfunction; transmitting a message from the source node to the destinationnode wherein the message comprises a first timestamp representing alocal time of transmission (t₁); receiving the message at thedestination node and stamping the message with a second timestamp thatis indicative of a local time of reception (t₂); calculating adifference between the first timestamp and the second timestamp; andcalibrating a clock of the destination node by compensating for clockdrift between the source node and the destination node using thecalculated difference.
 2. The method of claim 1 further comprising:repeating the method at least once; averaging the calculateddifferences; and calibrating the clock of the destination node bycompensating for the clock drift between the source node and thedestination node using the average of the calculated differences.
 3. Themethod of claim 1 further comprising: sending a reply message to thesource node from the destination node wherein the reply messagecomprises the first timestamp, the second timestamp and a thirdtimestamp, where the third timestamp represents a local time oftransmission for the reply message (t₃); receiving the reply message atthe source node; determining a local time of reception for the replynode at the source node (t₄); computing a range between the source nodeand the destination node using the times t₁, t₂, t₃, and t₄; using adirectional antenna to determine an approximate area within which thedestination node is located; and determining an approximate location ofthe destination node within the approximate area located at the rangefrom the source node.
 4. The method of claim 3 wherein the directionalantenna comprises a plurality of radiation elements that each form aradiation pattern and wherein the approximate area is within one of theradiation patterns.
 5. The method of claim 3 further comprising:computing a plurality of ranges to a plurality of nodes using aplurality of directional antennas to improve the accuracy of theapproximate location.
 6. The method of claim 5, wherein the directionalantenna is omni-directional and the method further comprises: computinga location of nodes using a plurality of omni-directional antennas and atriangulation computation.
 7. An apparatus for calibrating a clock of anode comprising: a source node for transmitting a message to adestination node wherein the message comprises a first timestamprepresenting a local time of transmission (t₁); a destination node forreceiving the message and stamping the message with a second timestampthat is indicative of a local time of reception (t₂); a processor withinthe destination node for calculating a first difference between thefirst timestamp and the second timestamp; the source node transmittingat least a second message to the destination node wherein the at least asecond message comprises a first timestamp representing a local time oftransmission (t₁); the destination node for receiving the at least asecond message and stamping the at least a second message with a secondtimestamp that is indicative of a local time of reception (t₂); aprocessor within the destination node for calculating at least a seconddifference between the first timestamp and the second timestamp; theprocessor calculating an average between the first difference and the atleast a second difference; and the processor calibrating the clock ofthe destination node using the average.
 8. The apparatus of claim 7further comprising: the destination node for sending a reply message tothe source node wherein the reply message comprises the first timestamp,the second timestamp and a third timestamp, where the third timestamprepresents a local time of transmission for the reply message (t₃); adirectional antenna, coupled to the source node, for creating aplurality of radiation patterns, where each radiation pattern extendsfrom the antenna in a unique direction; and a processor within thesource node for determining a local time of reception for the reply nodeat the source node (t₄), computing a range between the source node andthe destination node using the times t₁, t₂, t₃, and t₄ and determininga radiation pattern used to receive the message from the destinationnode, where the determined radiation pattern defines a direction.
 9. Theapparatus of claim 8 wherein the directional antenna has anomni-directional radiation pattern.
 10. The apparatus of claim 8 furthercomprising: the processor for computing a range between the source nodeand the destination node using the times t₁, t₂, t₃, and t₄, using thedirectional antenna to determine an approximate area within which thedestination node is located and determining an approximate location ofthe destination node within an approximate area located at the rangefrom the source node.
 11. The apparatus of claim 10 wherein theapproximate area within which the destination node is located is withinthe determined radiation pattern.
 12. The apparatus of claim 7, whereinthe source node and the destination form at least a portion of a meshnetwork.
 13. A computer-implemented method for determining the locationof a node comprising: transmitting a message from a source node to adestination node wherein the message comprises a first timestamprepresenting a local time of transmission (t1); receiving the message atthe destination node and stamping the message with a second timestampthat is indicative of a local time of reception (t2); sending a replymessage to the source node from the destination node wherein the replymessage comprises the first timestamp, the second timestamp and a thirdtimestamp, where the third timestamp represents a local time oftransmission for the reply message (t3); receiving the reply message atthe source node; determining a local time of reception for the replynode at the source node (t4; computing a range between the source nodeand the destination node using the times t1, t2, t3, and t4; using adirectional antenna to determine an approximate area within which thedestination node is located; and determining an approximate location ofthe destination node within the approximate area located at the rangefrom the source node.
 14. The method of claim 13 wherein the directionalantenna comprises a plurality of radiation elements that each form aradiation pattern and wherein the approximate area is within one of theradiation patterns.
 15. The method of claim 13 further comprising:computing a plurality of ranges to a plurality of nodes using aplurality of directional antennas to improve the accuracy of theapproximate location.
 16. The method of claim 13 further comprising:synchronizing the destination node with the source node using a standard802.11 time synchronization function; calculating a difference betweenthe first timestamp and the second timestamp; and calibrating a clock ofthe destination node by compensating for clock drift between the sourcenode and the destination node using the calculated difference.
 17. Themethod of claim 16 further comprising: repeating the method at leastonce; averaging the calculated differences; and calibrating the clock ofthe destination node by compensating for the clock drift between thesource node and the destination node using the average of the calculateddifferences.